1. Field of the Invention
The present invention relates to a successive cold working process for successively forging a billet below a transformation temperature (i.e., a recrystallization temperature) thereof.
2. Description of the Relevant Art
One conventional process for successively working a billet through a plurality of forging steps (pressing steps) is disclosed in Japanese Laid-Open Patent Publication No. 60-115343.
The disclosed process works the billet into an anchor bolt with a transfer press machine having a plurality of forging dies arranged side by side.
Japanese Laid-Open Patent Publication No. 59-220243 shows a cold forging process for forging a billet between a die and a punch below a transformation temperature of the billet.
The above prior methods however do not take into account the problems of lubrication and age hardening (work hardening).
When cold-forging a billet by extrusion or upsetting, galling would occur between the billet and a die unless a lubricating film were present on the surface of the billet. To prevent such galling, a film such as a phosphate coating is formed on the billet surface by bonderizing the billet as is well known in the art. Alternatively, lubricating oil is applied to the billet or the die in each forging step as disclosed in Japanese Laid-Open Utility Model Publication No. 55-60238.
The thickness of a chemically converted lubricating film on a bonderized billet is however largely reduced in a single forging step irrespective of the desired forging ratio, and therefore the lubricating film does not allow the billet to be forged in successive steps. Where the forging ratio is high, the lubricating effect of the film is lost by the heat produced when the billet is forged. Accordingly, use of such a bonderized coating is not suitable in a transfer successive forging process. The application of lubricating oil is disadvantageous in that it cannot be uniformly coated on the entire surfaces of the billet and the die, a large amount of lubricating oil is needed, and applied lubricating oil tends to be scattered out of the die, thus contaminating the working environment. The coated lubricating oil cannot be processed easily. Since the lubricating oil film is simply present on the billet surface unlike the chemically converted coating, the billet surface may lose such lubricating oil and cause galling if the forging pressure is increased.
According to another lubricating method disclosed in Japanese Laid-Open Patent Publication No. 59-150639, lubricating oil is supplied under pressure into a region below a billet from the bottom of a die while the billet is being forged under pressure. The disclosed method is however not appropriate for use in successive cold forging because the method is primarily aimed at achieving the effect of a double-acting press by applying a force to the lower surface of the billet in counteracting the pressure of a punch.
When a billet is forged below a transformation temperature (recrystallization temperature), the billet is subject to age hardening. The inventor has found that the age hardening manifests itself upon elapse of a certain period of time after the billet has been forged. In the conventional processes, a lubricating coating is formed on the billet surface prior to each forging step, and hence age hardening progresses while the lubricating coating is being formed. Consequently, a next forging step cannot be effected before the billet undergoes a high degree of age hardening. It has therefore been necessary to anneal the billet prior to each of the second and subsequent forging steps, which however results in a poor production efficiency.
Japanese Patent Publication No. 63-41665 discloses a cold forging process in which certain condition are selected for upsetting and rearward extrusion processes, and these upsetting and rearward extrusion processes are successively carried out within a very short period of time, thus omitting lubrication and intermediate annealing steps from between these upsetting and rearward extrusion processes. According to this prior art, however, various forging conditions such as a billet size, a billet shape, a forging ratio, a carbon equivalent, and the like are extremely limited.